Transponders are transmitters/receivers which normally do not need their own energy supply. Transponders are addressed by an electromagnetic high-frequency field and obtain their operating energy through rectification of the high-frequency field. They are usually configured to perform a particular function in response to a particular receive signal. This function may e.g. be that they in turn transmit an identification code, that they write into an assigned memory, or that they transmit some other signal, which can in turn be received by a receiver and which provides the same with information on a particular property of the transponder.
In recent times very cheap transponders have appeared on the market, which can e.g. be incorporated into labels so as to identify an object which has the label positioned on or stuck to it. Distributing the mail is one application which may be thought of here. Although an address which a person can read is stuck onto parcels, the actual distribution can nevertheless be performed by means of a transponder housed in the label and which can be read by a suitable reading device. Such a label with an electronic transponder becomes superfluous when the parcel has been delivered and the label is more or less broken. Such transponders must be very cheap since they are one-use articles.
In the above mail distribution scenario a transponder affixed to a parcel can be initialized at the outset in order e.g. to indicate a special parcel by means of a serial number. Alternatively, the transponder could issue the destination address of the parcel in response to a certain query data sequence. Such a parcel will, if it is submitted at a post office, pass through several distribution centres on its way to the recipient. As an example of writing into the transponder, the transponder assigned to the parcel via the label could have a certain code written into it at each distribution centre so as to follow the path of the parcel. An example of writing into a transponder is, of course, also the initialization at the start, i.e. the original input of the destination address.
In a parcel distribution centre lots of parcels with transponders are transported on the conveyor belt. A reading device can activate the transponders of the individual parcels so as to determine their destination address in order to send them in the right direction from the main conveyor belt.
Such transponders are mainly used as identification systems. Inexpensive identification transponders of this kind, e.g. incorporated in labels, can be used wherever particular objects are to be identified automatically.
Some boundary conditions exist for the design of antenna arrangements for transponder read units. On the one hand an antenna arrangement must generate a sufficiently large field density so that the transponder can "draw" enough electrical energy from the antenna field. In addition, there are certain frequency bands which have been allocated for such radio systems. Finally, an important boundary condition for such antenna arrangements is that the dimensions e.g. of the conveyor belt or also of the objects equipped with transponders are rigidly fixed in advance and there is no possibility of modifying the objects or the conveyor equipment to match the antenna geometries; instead, the antenna geometries must be matched to existing conveyor belts and existing object sizes. To stay with the example of parcel distribution, it must be possible to process parcels even though they may be 90 cm high. The parcel size cannot be changed because of the antenna geometry, rather the antenna geometry must necessarily accommodate itself to the parcel size and also to existing conveyor belt equipment. Furthermore, the labels can quite generally be located anywhere on the parcel.
FIG. 7 shows a known embodiment of an antenna arrangement, together with the circuitry, for reading out transponders on objects on a conveyor belt or for communicating with the same. The arrangement comprises a transponder read unit 100, which supplies electrical signals to be sent to the transponders and which receives the electrical signals from the transponders. This takes place via a three-port circuit 102, which has a first matching network 104 for a first frame antenna 106 connected to one of its two output ports and a second matching network 108 for a second frame antenna 110 connected to the other output port. In the known arrangement the two frame antennas 106 and 110 are arranged to the left and to the right of a conveyor direction 112, which is symbolized by a broken arrow in FIG. 7. In order that high and large parcels can be used in the arrangement shown in FIG. 7, the two individual frame antennas must have considerable dimensions.
Furthermore, it must be taken into account that the geometry of the conveyor belt, which is shown symbolically by the directional arrow 112, is also fixed in advance and can also assume considerable proportions. In order that transponders in the middle of the conveyor belt can also be read out, the antenna must be supplied with sufficient energy. In particular, the field strength must be restricted to a permissible maximum, which is specified by national authorities of the country in which the system is operated.
Furthermore, it is necessary that the two matching networks 104 and 108 are closely matched to one another in order that the transponder signals can be received correctly and can be evaluated without substantial error by the transponder read unit 100.
The following is a common approximation formula for the inductance of a frame antenna which should be valid for the frame antennas 106 and 110. EQU L=2.multidot.u.multidot.( ln (u/D)-Kq).multidot.(N).sup.1.8
The individual parameters in this equation have the following significance:
u Circumference of the antenna loop [cm] PA1 D Width or diameter of the conductor [cm] PA1 Kq Correction factor which takes account of the shape of the antenna (Kq=1.47 for square antennas, Kq=1.07 for circular antennas) PA1 N Number of turns PA1 L Antenna inductance [nH]
The resonant frequency is given by the following equation: EQU f=1/(2.multidot..pi..multidot.C.multidot.L)
An increase in the circumference of a frame antenna leads, as can be seen from the equation for the antenna inductance, to an increase in the inductance. Because of the logarithmic function, however, the rise in the inductance can only be partially counteracted by increasing the width of the conductor. Furthermore, increasing the number of turns also leads to an increase in the inductance.
A high inductance is not desirable for stable antenna operation, however, if the frame antennas are to be operated at frequencies above 10 MHz. From the equation for the resonant frequency it becomes clear that for very large antennas the antenna capacitance becomes very small, typically less than 100 pF, in order to achieve the required operating frequency. Since stray capacitances and the self-capacitance of the inductance can already reach this value, however, a stable configuration and tuning of such a resonant circuit is no longer possible. In the case of transponder read units in particular, the stray capacitances present a considerable problem since, on the one hand the antennas must be large and on the other hand a multitude of continuously varying stray capacitances exist in a mail distribution centre next to a conveyor belt, on the one hand due to objects of different sizes and on the other hand due to the continuously varying environment. To become independent of the stray capacitances the antenna must be loaded with a capacitance which is considerably higher than the stray capacitances in such a way that variable stray capacitances only have negligible effects on the antenna operation. However, if the inductance of the antenna were already very large, it would no longer be possible to connect a very large capacitance to the antenna to achieve a particular operating frequency in the range between 10 and 20 MHz. Thus the requirement exists that the inductance of the antenna should be kept as small as possible so that a precisely defined capacitance of the highest possible value can be included in the antenna circuit.
In addition, such antenna arrangements should not be too costly and be simple to operate so that people who are not accredited specialists can also operate such an installation and even carry out minor tuning operations.